This invention relates in general to sewing machines and in particular to a new and useful thread cutter for a zig-zag sewing machine which includes a catcher member having barbs on respective side edges which engage portions of the legs of the needle loop and move it into association with a cutting knife.
A thread cutter is known from German Utility Model No. 1,968,920. For cutting off the thread, the sewing machine is stopped first at the end of a seam with the needle in a bottom position. In zig-zag sewing machines it is generally left to chance whether the needle is stopped in the left or right overstitch position of the needle. Then the sewing machine is started at low speed for a half revolution of the arm shaft until the needle is in a top position. The looper shaft with the looper performs a complete revolution. In the course of this one revolution, the looper tip engages the needle loop formed during the ascending movement of the needle in order to widen it and to conduct it completely around the bobbin case with the looper thread supply. After the looper tip has engaged the needle thread loop and widened it partly, the thread catcher, which has on one side a recess with a barb, is moved from its starting position against the force of a return spring beyond the path of motion of the needle. Its tip penetrates the needle thread loop, and the leg of the needle thread loop leading to the sewing material and the looper thread are guided by the thread catcher to the side with the recess, while the leg of the needle thread loop leading to the needle is deflected to the other side of the thread catcher.
In the further course of the movement of the looper and of the thread catcher, the needle thread loop is conducted completely by the looper around the bobbin case. The thread catcher penetrates so far into the needle thread loop until the leg of the needle thread loop leading to the sewing material together with the looper thread, has arrived in the recess behind the barb. The recess is so arranged in the thread catcher of this device that the thread sections to be cut off are positively separated by the catcher tip during the piercing of the needle in the overstitch position farthest away from the thread catcher, and is still positively caught by the end of the stroke of the thread catcher. The looper thread caught in the recess behind the barb, and the leg of the needle thread loop leading to the sewing material are fed to a stationary counterknife during the return movement of the thread catcher by the return spring relaxing after its driving means has been shut off, and are cut off.
Since the needle has in one overstitch position a greater distance from the stationary counterknife than in the other, and the stroke of the thread catcher remains unchanged, the threads are necessarily cut off, after the machine has been stopped, in one overstitch position in a different distance from the last puncture of the needle than after the stoppage in the other overstitch position. For the thread ends remaining on the sewing material, there are no adverse effects from the different length resulting from the stitch position. But the reliability of the first stitch formation of a new seam following the thread cutter depends on the length of the thread end leading to the needle. Heretofore it as not possible to meet the requirement for optimally short thread ends in both overstitch positions of the needle, because the size of the catcher movement and the arrangement of the stationary counterknife depend on the result of the thread cutoff in the overstitch position of the needle closer to the thread catcher, in which the length of the needle thread end, after the thread cutoff in this overstitch position, must suffice for the first stitch formation of the new seam in order to avoid time losses caused, e.g. in an unsuccessful approach because of a too short thread end by the stoppage of the machine and the rethreading, and other disadvantages, like perforation of the sewing material by the needle. The stationary counterknife therefore had to be arranged in a greater distance from the stitch hole than it was desirable to obtain optimally short thread ends on the sewing material.
When the thread was cut off after the stoppage of the machine in the overstitch position farther away from the thread catcher, it was necessary to put up with longer thread ends on the sewing material.